The invention relates generally to optical sensor technologies. In particular, the invention relates to optical sensors isolated from environmental elements in relatively small diameter housing.
Available electronic sensors measure a variety of values, such as, pH, color, temperature, or pressure, to name a few. For systems that require a string of electronic sensors over a long distance, e.g. twenty to thirty kilometers or longer, powering the electronic sensors becomes difficult. Conventionally, the powering of electronic sensors requires running electrical wire from a power source to each of the electronic sensors. Powering electronic sensors electrically has been unreliable in the petroleum and gas industry. For example, electric wires spanning long distances are subject to a significant amount of interference and noise, thereby reducing the accuracy of the electronic sensors.
Optical fibers have become the communication medium of choice for long distance communication due to their excellent light transmission characteristics over long distances and the ease of fabrication of lengths of many kilometers. Further, the light being transmitted can interrogate the sensors, thus obviating the need for lengthy electrical wires. This is particularly important in the petroleum and gas industry, where strings of electronic sensors are used in wells to monitor downhole conditions.
As a result, in the petroleum and gas industry, passive fiber optic sensors are used to obtain various downhole measurements, such as, pressure or temperature. A string of optical fibers within a fiber optic system is used to communicate information from wells being drilled, as well as from completed wells. The optical fiber could be deployed with single point pressure-temperature fiber optic sensor. Also, a series of weakly reflecting fiber Bragg gratings (FBGs) may be written into a length of optical fiber or a single point Fabry-Perot sensor may be spliced into a length of optical fiber. An optical signal is transmitted down the fiber, which is reflected and/or scattered back to a receiver and analyzed to characterize external parameters along the length of the optical fiber. Using this information, downhole measurements including but not limited to temperature, pressure, and chemical environment may be obtained.
Known optical sensor geometries include Fabry-Perot, Bragg-grating, Mach-Zehnder, Michelson and Sagnac, among others. If all of the sensing occurs within the optical fiber, the optical sensor is an intrinsic fiber; therefore, the fiber acts as both a transmission medium and a sensing element. If the fiber does not act as a sensing element but merely acts as a transmission medium, the optical sensor is classified as an extrinsic sensor. In an extrinsic optical sensor, the optical fiber transmits the light source to an external medium, for example air, where the light is modulated to provide the desired sensing or detection. Optical sensors are also classified by the optical principle which they operate. Interferometric optical sensors utilize interference patterns between source light beams and reflected beams. Intensity based sensors measure the light lost from the optical fiber.
One type of optical sensor is the extrinsic Fabry-Perot interferometer (“EFPI”). An EFPI utilizes two reflective surfaces and the difference or shift between a reference beam and a reflected beam directed through an optical fiber. This phase shift is used to determine or calculate the desired physical or environmental characteristic.
However, when conventional optical fibers such as germanium-doped silica fibers are exposed to the intense heat, pressure, and chemical-rich environment of an oil well attenuation losses increase. This increase in the loss of optical strength of the signal is due, in part, to the diffusion of hydrogen into the glass structure. Hydrogen atoms bond to any open or weak bonds in the glass structure, such as to certain germanium atoms in the vicinity of germanium-oxygen deficient centers or to form SiOH and/or GeOH. For germanium doped fibers, the attenuation increases rapidly with increases in temperature. As temperatures in a typical oil or gas well generally range from slightly less than surface temperature near the surface to between about 90 to 250 degrees Centigrade (C.), conventional germanium-doped optical fibers are generally not sufficiently stable for prolonged use at depth in a well. While coating germanium-doped silica fibers with carbon or similar molecularly dense materials is an effective way to reduce hydrogen diffusion into the glass at lower temperatures, such as below 120 degrees C., the effectiveness of the carbon coating diminishes rapidly as the ambient temperature increases.
One known way to protect sensor fiber from the harsh conditions of the well is to physically isolate the fiber from the environmental conditions of the well, which is described in WIPO PCT Publication WO 2005/024365 A2, which is incorporated herein by reference in its entirety. The fiber of the sensor is typically placed within a protective housing such as a hollow metal cylindrical housing. The interior chamber of the housing includes a compressible clean fluid which is capable of translating the pressure and/or temperature of the well conditions. At the interface of the housing and the well conditions is a deformable diaphragm which forms one wall of the interior chamber of the housing. The diaphragm is deflected by the external pressure, thereby varying the volume of the interior chamber and compressing the clean fluid therewithin. As such, the external pressure is translated to the optical sensor even though the optical sensor is not exposed to the harsh environmental conditions within the well. Similarly, the heat transfer properties of the diaphragm and the clean fluid may be selected to allow the optical sensor to detect the downhole temperature. In this manner, the optical sensor is protected from the chemicals and impurities within the well while still obtaining useful pressure and temperature readings.
One drawback to the currently available isolated optical sensors as described above is that the diameter of the housing is relatively large to provide the diaphragm with sufficient surface area to move freely to translate the ambient pressure to the isolated sensor. Consequently, isolated sensors are not available for many situations in tight spaces, a situation that often arises in downhole and other pumping applications. Therefore, a need exists for isolated optical sensors that are capable of being made smaller than those available in the art.